Access to a formation for fracturing is typically obtained with a series of sliding sleeves. The sleeves can come with a ball seat so that in some completions there can be as many as 40 balls of a gradually increasing diameter that need to be dropped in a specific order for opening the sleeves in a specific order going from downhole to uphole.
More recently Baker Hughes has provided sliding sleeves with articulated ball seats so that a ball can land on a seat and shift a sliding sleeve and then the seat can open to allow the ball to pass further downhole to the hole bottom or to another ball seat on another sliding sleeve. This design is called the Frac Point MP Sleeve.
Another design uses tubing pressure to shift a sleeve. Opposed and isolated atmospheric chambers are provided on the outside of the sliding sleeve. One of the atmospheric chambers has a rupture disc for access of tubing pressure at a predetermined value into one of the atmospheric chambers. When the rupture disc breaks the sliding sleeve is shifted because one of the atmospheric chambers now has tubing pressure and on the opposite side of a piston formed onto the outside of the sliding sleeve there is still atmospheric pressure. Different sleeves have different rupture disc pressure ratings and in that manner the sleeves can be shifted in a hoped for predetermined order. The risk in this system is that rupture discs sometimes have significant variability in their burst pressure so that a sleeve may shift at a time where it was not planned for it to shift.
Also related for general background on sliding sleeves are U.S. Pat. Nos. 7,325,617 and 7,552,779. U.S. Pat. No. 5,301,755 shows the use of a low pressure chamber to set off a perforating gun. U.S. Pat. No. 7,150,318 shows a tractor powered shifting tool that releases a cup seal so that when engaged to a sleeve pressure can be supplied against the open cup seal to drive the shifter and take the sliding sleeve with it.
The present invention improves on the prior designs by providing a ball seat that shifts with applied differential pressure to release the ball to go further downhole to the next ball seat and the act of shifting the ball seat opens one atmospheric chamber on one side of a piston integrated into the sliding sleeve to tubing pressure. Since the other side of the piston is still at atmospheric pressure, the sliding sleeve is moved to a travel stop by pressure differential now acting on the integrated piston associated with the sliding sleeve. The process is repeated until all the sleeves have shifted and each port opened by the sleeve shifting has been used as an access location to fracture the formation. The fracturing can take place even if there is cement outside the port opened by the shifting sleeve. These and other aspects of the invention will be more readily understood by those skilled in the art from a review of the detailed description and the associated drawings while understanding that the full scope of the invention is to be found in the appended claims.